In recent years, switching power supplies have been widely used in response to the needs for miniaturization of electronic equipment and higher power conversion efficiency of electronic equipment. In switching power supplies, after commercial AC power is rectified and smoothed to DC power, the power is converted to RF power by a switching operation of a semiconductor device having a high breakdown voltage, the power is transferred by a small transformer for power conversion, and the transferred power is rectified and smoothed to obtain low voltage DC power.
Further, consciousness about the issue of energy conservation has risen worldwide. Against this backdrop, electronic equipment with lower standby consumption has been demanded and thus techniques for reducing standby power consumption during the use of switching power supplies have been developed.
Moreover, in a switching power supply, an input side and an output side are electrically insulated from each other for safety by a power conversion transformer, and thus in order to control an output voltage Vo, the following technique has been widely used: the output voltage Vo on the secondary side is detected by a secondary-side output voltage detection circuit provided on the secondary side, a signal indicating the detection is transferred from the secondary side to the primary side through a photocoupler, and the output voltage Vo is controlled based on the transferred signal.
However, the provision of secondary-side output voltage detection circuits and photocouplers has interfered with miniaturization of switching power supplies. Further, in small switching power supplies, the provision of secondary-side output voltage detection circuits and photocouplers increases the cost. For this reason, a technique called auxiliary winding feedback has been conventionally used in stead of secondary-side output voltage detection circuits and photocouplers. To be specific, in this technique, a voltage generated on an auxiliary wining provided on the primary side is the turns ratio times a voltage generated on a secondary winding. The voltage generated on the auxiliary winding is rectified and smoothed to obtain a voltage substantially proportionate to an output voltage Vo, and the output voltage Vo is controlled based on the obtained voltage.
A control method for stabilizing the output voltage of a switching power supply includes PWM control for controlling output voltage by changing a ratio (duty ratio) between the on time and the off time of a switching element, and PFM control for controlling output voltage by changing the off time of a switching element while fixing the on time of the switching element. To be specific, in PWM control, when a load increases and an output voltage Vo decreases, the on time of a pulse for driving a switching element is increased. On the other hand, in PFM control, when a load decreases and an output current Io also decreases, the frequency of a pulse for driving a switching element is reduced.
A mode of controlling the driving of a switching element thus based on a pulse includes: current mode of controlling the driving of the switching element based on both of the detection of an output voltage and the detection of a drain current passing through the switching element, and voltage mode of controlling the driving of the switching element based on the detection of an output voltage.
As an example of a conventional switching power supply, the following will describe a switching power supply of auxiliary winding feedback type for performing PWM control in the current mode. FIG. 10 is a block diagram showing the conventional switching power supply.
In FIG. 10, a switching element 1 is a power MOSFET. The switching element 1 has three terminals of a DRAIN terminal acting as an input terminal, a SOURCE terminal acting as an output terminal, and a GATE terminal acting as a control terminal. The switching element 1, in response to a control signal received on the control terminal, oscillates so as to electrically couple or decouple the input terminal and the output terminal. Further, the switching element 1 switches, according to the oscillating operation (switching operation), a DC voltage supplied to a primary winding 110A of a transformer 110 for power conversion. The switching operation of the switching element 1 generates a pulse voltage on a secondary winding 110B and an auxiliary winding 110C.
A control circuit 100 is formed on the same semiconductor substrate. The control circuit 100 controls the switching operation (oscillating operation) of the switching element 1 by generating a control signal and transmitting the signal to the control terminal of the switching element 1. Further, the control circuit 100 has, as external connection terminals, three terminals of the input terminal (DRAIN terminal) of the switching element 1 an auxiliary power supply voltage input terminal (VCC terminal), and a GND terminal (SOURCE terminal) acting as the output terminal of the switching element 1.
The transformer 110 has the primary winding 110A, the secondary winding 110B, and the auxiliary winding 110C. The primary winding 110A and the secondary winding 110B are opposite in polarity and the switching power supply is a flyback power supply. The secondary winding 110B and the auxiliary winding 110C have the same polarity, and a voltage generated on the auxiliary winding 110C is proportionate to a voltage generated on the secondary winding 110B. The control circuit 100 detects the output voltage Vo of the secondary side by using the voltage generated on the auxiliary winding 110C.
A rectifying/smoothing circuit including a diode 120 and a capacitance 121 is connected to the auxiliary winding 110C. The rectifying/smoothing circuit is used as the auxiliary power supply voltage generation circuit of the control circuit 100. To be specific, the rectifying/smoothing circuit generates an auxiliary power supply voltage VCC by rectifying and smoothing a pulse voltage generated on the auxiliary winding 110C in response to a switching operation of the switching element 1, and supplies the voltage VCC to the VCC terminal.
A rectifying/smoothing circuit including a diode 130 and a capacitance 131 is connected to the secondary winding 110B. The rectifying/smoothing circuit is used as the output voltage generation circuit of the switching power supply. To be specific, the rectifying/smoothing circuit generates an output voltage Vo by rectifying and smoothing a pulse voltage generated on the secondary winding 110B in response to a switching operation of the switching element 1, and supplies the output voltage Vo to a load 132.
In the control circuit 100, a regulator 2 is connected to the VCC terminal and the DRAIN terminal. The regulator 2 supplies a current from one of the DRAIN terminal and the VCC terminal to an internal circuit power supply VDD of the control circuit 100 and stabilizes the voltage of the internal circuit power supply VDD at a constant value.
In other words, before the start of the switching operation of the switching element 1, the regulator 2 supplies the current from the DRAIN terminal to the internal circuit power supply VDD and simultaneously supplies the current from the DRAIN terminal to the capacitance 121 of the auxiliary power supply voltage generation circuit via the VCC terminal to increase the voltages of the auxiliary power supply VCC and the internal circuit power supply VDD.
After the start of the switching operation of the switching element 1, the regulator 2 stops supplying current from the DRAIN terminal to the VCC terminal. To be specific, when the auxiliary power supply voltage VCC is not lower than the constant value, the regulator 2 supplies, from the VCC terminal to the internal circuit power supply VDD, a current based on the auxiliary power supply voltage VCC. By supplying the circuit current of the control circuit 100 thus from the auxiliary winding 110C, power consumption is effectively reduced.
The VCC terminal acts as a current source of the control circuit 100 and simultaneously acts as a control terminal of feedback control. In other words, the VCC terminal is connected to the regulator 2 and a feedback circuit 3.
The feedback circuit 3 is made up of an OP amplifier 4, a resistor 5a, a resistor 5b, and a resistor 5c. The resistors 5a and 5b divide the voltage of the VCC terminal (auxiliary power supply voltage VCC) and supply the voltage to the inverting input terminal of the OP amplifier 4. The resistor 5c connected between the inverting input terminal and output terminal of the OP amplifier 4 determines the amplification factor of the feedback circuit 3.
The feedback circuit 3 compares the auxiliary power supply voltage VCC and the reference voltage value, generates an error amplification signal VEAO based on a voltage difference, and transmits the signal VEAO to a drain current control circuit 7.
In the switching power supply of auxiliary winding type for performing such PWM control, the peak value of current (drain current) ID passing through the switching element 1 is controlled according to the signal level of the error amplification signal VEAO, so that the output voltage Vo is stabilized.
Moreover, in a switching power supply for performing PFM control, the frequency of a clock signal generated by an oscillator is controlled according to the signal level of an error amplification signal from a feedback circuit to control the oscillatory frequency of a switching element, so that an output voltage Vo is stabilized. To be specific, in the switching power supply for performing PFM control, when a load increases and the signal level of the error amplification signal also increases, the frequency of the clock signal is increased.
A drain current detection circuit 6 detects the current ID passing through the switching element 1, generates an element current detection signal VCL serving as a voltage signal corresponding to the current value, and transmits the signal VCL to a drain current control circuit 7.
The drain current control circuit 7 is fed with an overcurrent protection reference voltage VLIMIT serving as the reference voltage and the error amplification signal VEAO from the feedback circuit 3. Further, when the signal level of the element current detection signal VCL from the drain current detection circuit 6 reaches the lower one of the signal level of the overcurrent protection reference voltage VLIMIT and the signal level of the error amplification signal VEAO, the drain current control circuit 7 generates a signal for determining the turn-off of the switching element 1 and transmits the signal to a latch circuit 9.
An oscillator 8 generates a clock signal having a fixed period for determining the turn-on of the switching element 1 and outputs the clock signal to the latch circuit 9.
The clock signal from the oscillator 8 is supplied as the reset input to the latch circuit 9 and the signal from the drain current control circuit 7 is supplied as the reset input to the latch circuit 9. The latch circuit 9 generates, from the set to the reset, a signal for turning on the switching element 1. In other words, the turn-on of the switching element 1 is controlled by the clock signal from the oscillator 8 and the turn-off of the switching element 1 is controlled by the signal from the drain current control circuit 7.
A gate driver 10 generates a control signal for driving the switching element 1 based on the signal from the latch circuit 9.
A light load intermittent oscillation control circuit 11 stops/restarts the input of the clock signal from the oscillator 8 to the set terminal of the latch circuit 9 according to the signal level of the error amplification signal VEAO from the feedback circuit 3, so that the switching operation of the switching element 1 is stopped/restarted and the switching element 1 is intermittently oscillated.
In other words, when the signal level of the error amplification signal VEAO decreases to a light load detection level VEAO1 at a light load, the light load intermittent oscillation control circuit 11 stops the generation of the clock signal in the oscillator 8 to stop the oscillation of the switching element 1. When the oscillation of the switching element 1 is stopped, the output voltage Vo decreases and the signal level of the error amplification signal VEAO increases. However, the light load detection level has a hysteresis of ΔVEAO and the light load intermittent oscillation control circuit 11 stops the oscillation of the switching element 1 until the signal level of the error amplification signal VEAO reaches “VEAO1 + ΔVEAO”. When the signal level of the error amplification signal VEAO reaches “VEAO 1 + ΔVEAO”, the light load intermittent oscillation control circuit 11 restarts the generation of the clock signal in the oscillator 8 and restarts the oscillation of the switching element 1. As a result, the switching element 1 is intermittently oscillated at a light load, reducing a switching loss.
However, the load increases even when the voltage of the VCC terminal can be substantially fixed relative to the output current Io in the conventional switching power supply of auxiliary winding feedback type as shown in FIG. 11. As the output current Io increases, the output voltage Vo decreases. Moreover, in the conventional switching power supply of auxiliary winding feedback type, the output voltage Vo rapidly increases during the intermittent oscillation. These problems arise regardless of whether the control is PWM control or PFM control and regardless of whether the mode is the current mode or the voltage mode. The factors of these problems will now be described below.
FIG. 12 shows the waveforms of voltages generated on the secondary winding and the auxiliary winding. When the switching element is turned off, the voltage having a waveform shown in the upper part of FIG. 12 is generated on the secondary winding and the voltage having a waveform shown in the lower part of FIG. 12 is generated on the auxiliary winding. If the secondary-side diode is an ideal device having no resistance components, the voltage generated on the secondary winding has a rectangular wave. However, a voltage drop actually occurs due to the resistance components of the secondary-side diode and thus the voltage generated on the secondary winding has the rectangular waveform shown in the upper part of FIG. 12. The waveform of the voltage generated on the auxiliary winding is proportionate to the voltage generated on the secondary winding.
FIG. 13 shows the waveforms of voltages generated on the secondary winding and the auxiliary winding at a light load and a heavy load in PWM control.
In PWM control, the heavier load, the higher peak of secondary current Id2 passing through the secondary winding. Therefore, a voltage drop ΔVd2 determined by the product of a resistance component Rd2 of the secondary-side diode and the secondary current Id2 increases with the load.
On the other hand, the circuit current of the control circuit is supplied from the auxiliary winding, and thus the current also passes through the diode on the auxiliary winding side. Therefore, as shown in the lower part of FIG. 13, the voltage VCC having been rectified by the diode on the auxiliary winding side is dropped by ΔVCC from the peak voltage generated on the auxiliary winding. ΔVCC is determined by the product of a current Id1 passing through the diode on the auxiliary winding side and a resistance component Rd1 of the diode on the auxiliary winding side.
However, the circuit current of the control circuit is sufficiently small relative to the output current Io of the secondary side and even when the load fluctuates, the peak of the current Id1 passing through the diode on the auxiliary winding side does not fluctuate as greatly as the secondary current Id2. Thus the voltage drop ΔVCC hardly changes even when the load fluctuates.
In other words, when the load increases, the voltage drop ΔVCC hardly fluctuates but the voltage drop ΔVd2 increases. Further, the output voltage Vo has, as shown in the upper part of FIG. 13, a value obtained by subtracting “ΔVd2 + Vf2” from the peak voltage generated on the secondary winding. “Vf2” represents the forward voltage of the secondary-side diode.
Therefore, in the conventional switching power supply of auxiliary winding feedback type, the switching operation of the switching element is controlled so as to keep constant the voltage of the VCC terminal. Thus when the voltage drop ΔVCC hardly fluctuates, fluctuations of the voltage drop ΔVd2 caused by the resistance component of the secondary-side diode are directly reflected on the output voltage Vo, and the output voltage Vo decreases as the output current Io increases.
As described above, in the conventional switching power supply of auxiliary winding feedback type, it is not possible to prevent the output voltage Vo from changing with the load in PWM control, regardless of whether the mode is the current mode or the voltage mode.
PFM control will now be described below. In PFM control, the peak of current passing through the secondary-side diode remains constant regardless of the load, and thus the voltage drop ΔVd2 caused by the resistance component of the secondary-side diode remains constant regardless of the load.
However, when the load increases and the oscillatory frequency of the switching element increases, an amount of current supplied from the auxiliary winding to the VCC terminal in each period decreases, so that the voltage drop ΔVCC decreases with the increasing load. In other words, although the voltage of the VCC terminal hardly fluctuates, the peak voltage generated on the auxiliary winding decreases with the increasing load. On the secondary winding, a voltage proportionate to the voltage of the auxiliary winding is generated, and thus the peak voltage generated on the secondary winding decreases with the increasing load. Therefore, as described above, the voltage drop ΔVd2 caused by the resistance component of the secondary-side diode remains constant regardless of the load, so that the output voltage Vo decreases with the increasing load. The output voltage Vo has a value determined by subtracting “ΔVd2 + Vf2” from the peak voltage generated on the secondary winding.
As described above, in the conventional switching power supply of auxiliary winding feedback type, it is not possible to prevent the output voltage Vo from fluctuating with the load in PFM control, regardless of whether the mode is the current mode or the voltage mode.
Further, the transformer for power conversion generally has a leakage inductance. The leakage inductance resonates with the parasitic capacitance of the switching element, so that the voltage generated on the auxiliary winding has high-frequency ringing waveforms as shown in the lower parts of FIGS. 12 and 13. Thus the peak of the voltage generated on the auxiliary winding has sharper waveforms. The influence of the leakage inductance is not negligible in the conventional switching power supply of auxiliary winding feedback type.
In other words, both in PWM control and PFM control, the ringing waveform caused by the leakage inductance becomes smaller as the load increases, regardless of whether the mode is the current mode or the voltage mode. Thus the peak voltage generated on the auxiliary winding decreases, so that the peak voltage generated on the secondary winding also decreases and reduces the output voltage Vo.
In the case of intermittent oscillation for reducing a switching loss at a light load, the following problem arises: since the number of oscillations decreases at a light load, the longer oscillation stop period, the larger amount of current supplied from the auxiliary winding to the VCC terminal in each period. Further, the voltage drop ΔVCC of the diode on the auxiliary winding side increases and the voltage of the VCC terminal decreases. Therefore, in the conventional switching power supply of auxiliary winding feedback type, both in PWM control and PFM control, the output voltage Vo is intended to rapidly increase during oscillation at a light load in order to keep constant the voltage of the VCC terminal, regardless of whether the mode is the current mode or the voltage mode.
Moreover, in the conventional switching power supply of auxiliary winding feedback type, as the load decreases, the ringing waveform caused by the leakage inductance becomes larger. Thus the voltage drop ΔVCC caused by the resistance component of the diode on the auxiliary winding side increases and the voltage of the VCC terminal decreases, so that the output voltage Vo further rapidly increases.
In order to solve the problem of the output voltage Vo decreasing with the increasing load, Japanese Patent Laid-Open No. 7-170731 discloses a switching power supply of auxiliary winding feedback type for a PWM control method, which will be described below. FIG. 14 is a block diagram showing the switching power supply. The same members as those of FIG. 10 are indicated by the same reference numerals and the explanation thereof is omitted. In the switching power supply of FIG. 14, the internal reference voltage of a feedback circuit 3 is variable.
In FIG. 14, a resistor 29 for detecting a drain current converts a drain current ID to a voltage. A pulse voltage generated by the resistor 29 is supplied to a drain current control circuit 7.
Meanwhile, a resistor 30 and a capacitance 31 smooth the pulse voltage generated by the resistor 29 for detecting a drain current, and supply the voltage to an OP amplifier 32. The OP amplifier 32 amplifies a voltage signal generated by the resistor 29 to a predetermined multiple and transmits the signal to a reference voltage variable circuit 13.
The reference voltage variable circuit 13 changes the internal reference voltage of the feedback circuit 3 in response to the signal from the OP amplifier 32. To be specific, when the load and the drain current ID increase, the reference voltage variable circuit 13 increases the internal reference voltage of the feedback circuit 3. When the internal reference voltage of the feedback circuit 3 increases, an error amplification signal VEAO falls, the peak value of the drain current ID decreases, and the peak of current passing through a diode 130 on the secondary side decreases. This operation prevents an output voltage Vo from decreasing at a heavy load.
In this conventional switching power supply, however, when using an output voltage control method such as the PFM control method having a constant drain current, it is not possible to obtain the effect of correcting the output voltage Vo.
Further, an extremely high capacitance value is necessary for the capacitance 31 for smoothing the signal having been converted to a voltage by the resistor 29 for detecting a drain current. Thus when a control circuit 100 is formed on the same semiconductor chip, the capacitance 31 is added as an external component, thereby increasing the cost and interfering with miniaturization.
The operation of increasing the internal reference voltage of the feedback circuit according to an increase of the load is equivalent to the operation of reducing the peak value of the drain current ID according to an increase of the load. This operation is contradictory to the original control performed by the feedback circuit 3. Thus the conventional switching power supply cannot stably control the output voltage Vo when the load rapidly fluctuates.